1. Field of the Invention
The present invention relates to a filter device in which a delay element is connected in parallel with a band pass filter and to a duplexer that includes the filter device.
2. Description of the Related Art
In the related art, a variety of band pass filters are used in mobile communication devices such as cellular phones. In cellular phones and so forth, the frequency width between the passbands of a plurality of channels has been becoming smaller. Therefore, an increase in the attenuation in the vicinities of the passbands has been strongly demanded.
In Japanese Unexamined Patent Application Publication No. 62-261211, a filter device is disclosed in which a delay element is connected in parallel with a main filter. The delay element has a characteristic that it has substantially the same amplitude characteristic but a phase that differs by (2n−1)π (n is a positive integer) at a desired frequency inside an attenuation range of the main filter. Therefore, direct waves at the desired frequency cancel each other out and attenuation can be increased at that frequency.
In Japanese Unexamined Patent Application Publication No. 62-261211, the delay element is formed by a surface acoustic wave (SAW) filter of a transversal type, for example.
However, in the filter device described in Japanese Unexamined Patent Application Publication No. 62-261211, there is a problem in that the frequency range of desired frequencies at which attenuation is desired to be increased is narrow.